1. Field of the Invention
The present invention relates to symbol timing recovery in a method of demodulating a quadrature modulated signal, generally a quadrature amplitude modulated signal, and particularly a Quadriphase or Quadrature Phase Shift Keying signal (QPSK), Offset-QPSK signal, or Differential-QPSK signal. The demodulation method demodulator still more particularly relates to a .pi./4-DQPSK signal such as used in IS-136 mobile radio systems and PWT cordless phones as standardized by the TIA/EIA. Such .pi./4-DQPSK signals, besides phase modulation, typically exhibit relatively large amplitude variations, i.e., are non-constant envelope signals. The present invention further relates to a demodulator, a receiver, and a transceiver for such quadrature modulated signals. In principle, the demodulator is suitable for demodulating zero-IF or low-IF down-converted quadrature signals.
2. Description of the Related Art
In the U.S. Pat. No. 4,766,392, a quadrature demodulator is disclosed for angle modulated signals, for use in a zero-IF receiver, i.e., a receiver in which a received angle modulated signal is supplied to a quadrature mixer for down-converting the received angle modulated signal to base band quadrature signals, an in-phase signal and a quadrature signal, with a zero intermediate frequency. The demodulator is of a so-called DAM-type, Differentiate and Multiply, in which, in the data demodulation path differentiate and multiply means are provided which are comprised of a pair of differentiators and multipliers in the data path, outputs of the differentiators being coupled to inputs of the multipliers, and inputs of the differentiators being cross-coupled to further inputs of the multipliers. For obtaining an `ideal` FM demodulator, envelope determining means of the in-phase and quadrature signals are provided, the envelope determining means controlling a gain of a variable gain amplifier in the data path, the variable gain amplifier being coupled between a subtractor for subtracting output signals of the multipliers from each other, and a low pass filter at an output of which a demodulated signal is present. The envelope determining means, together with the variable gain amplifier, form a fast AGC (Automatic Gain Control) means. Were not the fast AGC means present, then the demodulator would not be suitable for demodulating modulated .pi./4-DQPSK signals, these signals being non-constant amplitude signals. Then, the demodulator would not properly work as an FM-demodulator because amplitude fluctuations in the received radio frequency angle modulated signal cause a distorted complex signal, following quadrature mixing. A fast AGC is not desirable, however, a fast AGC requiring a higher bandwidth and higher power consumption. Particularly in portable devices, high power consumption is undesirable. For .pi./4-DQPSK signal, at zero-IF, the differentiators should be high pass filters, such as RC-filters, with a high 3-dB cutoff frequency, e.g., 10-20 MHz, i.e., high bandwidth filters. The diffentiators should operate in a highly linear range, such not being the case is the distance between the frequency operating range of the differentiators and their 3-dB cutoff frequency is small. Was this the case, the resulting BER (Bit Error Rate) of the demodulator would be very poor.
Other known demodulators for quadrature modulated signals, such as frequency or phase modulated signals, comprise differential detection means instead of demodulation being performed by an `ideal` demodulator such as disclosed in the above U.S. Pat. No. 4,766,392. Without special measures, such demodulators with differential detection means are not very suitable for demodulating .pi./4-DQPSK signals or other non-constant envelope quadrature modulated signals because of a then non-optimal symbol timing recovery.
.pi./4-DQPSK (de-)modulation as such is described in the handbook "RF and Microwave Circuit Design for Wireless Communications", L. E. Larson, Artech House, Inc., 1996, pp. 193-204.
Furthermore, digital phase lock loop circuits (DPLL) as such are well known, such circuits being particularly suitable as a symbol timing recovery means in a quadrature demodulator.